Spinel glass-ceramics

Stryjak and McMillan (1978) demonstrated increased hardness of the recrystallized material (Table 5.9), testing the properties of spinel glass-ceramics from the Zn0-Al203-Si02 system, with Zr02 admixture as the crystallization nucleator, in combination with the microstructure of glasses... 

Figure 4-5 Spinel-enstatite glass-ceramic as a magnetic memory disk substrate, in comparison to a canasite disk and a nickel-spinel glass-ceramic.

Pure spinel glass-ceramics show other properties distinct from enstatite-spinel glass-ceramic. These include spinel glass-ceramics and are characterized by GTE of 3.4 X 10 K (0 C-300°C) and transparency in the visible spectrum. They are also suitable for the production of highly pol-ishable disk substrates (Fig. 4-5) (Pinckney 1999). 

Pinckney L.R., "Transparent, High Strain Point Spinel Glass-Ceramic," /. Non-Cryst. Solids, 255,171-77 (1999). 

Oxides. Although not widespread commercially, glass-ceramics consisting of various oxide crystals in a matrix of siUceous residual glass offer properties not available with mote common siUcate crystals. In particular, glass-ceramics based on spinels and perovskites can be quite refractory and can yield useful optical and electrical properties. 

Such inversion must occur to a considerable degree in the glass-ceramics Ac as evident from the appearance of additional lines due to E A2 emissions resulting from the modified Cr(III) energy levels in the low symmetry characterizing cases of inverted spinel sites. This behaviour in crystals was thoroughly studied by Mikenda et al. ° . ... 

Fig. 5. Time-resolved spectra of spinel-type glass-ceramics Ac excited at 680 nm and at room temperature. The gate width is 100 ps, D is the time delay...

The time-resolved spectroscopy of Cr doped spinel and petalite-like phase types glass-ceramics enables a more accurate distinction of the and T2 energy levels of Cr(III) and of the equilibrium between the population of two levels, see Fig. 5, which presents the time-resolved spectra of spinel type glass-ceramics excited at 680 nm at room temperature Glass-ceramics derived from boro-silicate glasses containing Cr(III) were also recently studied by laser spectroscopy and fluorescence-line narrowing. 

By comparing the emission spectra of Cr in glass-ceramics with that of Cr " in single crystals one finds that the tendency to form pairs is greater in the glass-ceramics. In mixed crystals of spinel the appearance of Cr " pairs is again significant. 

Before leaving the topic we should mention that this is not the only way to prepare glass-ceramics. Another group of these materials used for dentistry go by the label In-Ceram . The material is prepared as a porous crystalline ceramic (alumina, spinel, and zirconia have all been used) and then infiltrated with an La-rich glass. The resulting glass-infiltrated ceramic can be quite translucent, can be colored to match other teeth, and is mechanically tough (for a ceramic). It can be coated with porcelains for improved appearance. 

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